This invention relates to a system for cutting off the supply of fuel to the engine of an automotive vehicle in a specific operating region within the deceleration region of the engine.
By way of example, an internal combustion engine fuel supply control method disclosed in the specification of Japanese Patent Publication (KOKOKU) No. 53-42854 is known generally as one method of cutting off the supply of fuel to an engine in a specific operating region of the engine. In accordance with the fuel supply control method disclosed in this known art, fuel cut-off rpm of the engine is set to be higher than the fuel restoration rpm and, as a result, a prescribed width (hysteresis) is provided between the fuel cut-off rpm and fuel restoration rpm. By setting hysteresis in this manner, the occurrence of so-called "hunting" between the fuel cut-off state and fuel restoration state is prevented.
However, the hysteresis set is a prescribed, fixed value. Consequently, problems arise when it is attempted to apply this control method uniformly to a manual-type transmission, in which the output shaft of the engine and the wheels of the vehicle are mechanically connected, and to an automatic-type transmission, which has a torque converter connecting the engine output shaft and the vehicle wheels in a state that allows relative rotation between them. The aforementioned problems will now be described.
The original purpose of fuel cut-off control is to improve fuel economy, i.e. to reduce fuel consumption. Accordingly, it is required that the fuel cut-off region be set to have a large width in order to assure an improvement in fuel economy. Here the fuel restoration rpm must be set a prescribed width greater than the idling rpm in order to avoid stalling of the engine. To achieve this, it is preferred in view of improving fuel economy that the fuel cut-off rpm be set to an rpm slightly higher than the fuel restoration rpm in a state where the fuel cut-off rpm is extremely close to the fuel restoration rpm. In other words, hysteresis should be set small in order to improve fuel economy.
The fact that hysteresis is small does not lead to problems in a manual-type transmission in which the engine and wheels are connected in a state which does not allow slipping when the engine is in a cruising condition. However, problems do arise in an automatic-type transmission in which the engine and wheels are connected in a state which does allow the aforementioned slipping when the engine is in a cruising condition.
Specifically, when the engine rotational speed or rpm drops in a state where the fuel has been cut off, the supply of fuel is resumed at the moment engine rpm attains the fuel restoration rpm. This restoration of the supply of fuel is accompanied by a sudden rise in engine rpm. Since the hysteresis is set small, this rise in engine rpm causes engine rpm to again rise above the fuel cut-off rpm, as a result of which the fuel cut-off state is established. Accordingly, the supply of fuel is cut off and engine rpm begins to decline. Thus, the rotational speed of the engine exhibits the hunting phenomenon, which causes the driver to experience unease with regard to the state of engine drive.
If hysteresis is set wide in order to prevent such hunting, on the other hand, this will interfere with the attainment of the aforementioned objective, namely the improvement in fuel economy.
Another problem is a high manufacturing cost, since it would be necessary to manufacture a control system for the manual-type transmission and a different control system for the automatic-type transmission.